1. Field of the Invention
This invention relates to an improved surface mount connector for connecting an electronic module or package, such as a multi-chip module, to a printed circuit board.
2. Description of the Prior Art
An important trend in the electronics industry has been the increasing utilization of integrated circuits as individual components due to their relatively inexpensive cost, miniature size, and electrical dependability. Today it is common for hundreds of complex integrated circuits to be treated as discrete components by the design engineer, with such integrated circuits being appropriately packaged and electrically connected to their associated printed circuit boards. However, the small dimensions and tight tolerances of miniaturized electrical connectors preclude the use of holes in printed circuit boards for establishing mechanical, as well as electrical, connection between an electronic module and a printed circuit board.
Thus, in order to connect the densely packaged integrated circuit modules to an associated printed circuit board, very high density connectors utilizing surface mount technology ("SMT") have become prevalent in the industry. Such connectors comprise a plurality of small electronic terminals disposed within a housing and soldered to a printed circuit board or the like. The densely packed leads or contact pins of an electronic module are disposed into contacting engagement with the electronic terminals for establishing electrical interconnection with the printed circuit board.
The terminals of the electrical surface mount connectors are presently soldered to the conductive pads of a printed circuit substrate using known soldering techniques such as solder paste reflow, wave soldering, infra-red soldering, or vapor phase reflow. However, due to the extremely small pitch distances between adjacent leads of electronic modules and the terminals of the associated surface mount connectors, these solder techniques are known to produce undesirable defects in the solder joint, such as a solder bridge between adjacent terminals, a cold joint, or solder contamination of the contact area of the terminal.
Due to the extremely small pitch distance between adjacent terminals, pitch distances of 0.1 mm are desirable, these known solder defects often occur in combination and success in preventing one defect often results in an increased occurrence of another. For example, since a cold joint defect is a lack of solder connection between a connector terminal and a printed wiring board pad, this problem is treated by applying more solder to the connection area. However, since adjacent terminals are densely packed together, the increased flow of solder and the resulting surface tension of two adjacent pools of solder cause a solder bridge to form between adjacent terminals such that the terminals become electrically connected when the solder solidifies, thus causing a short circuit. In order to obviate the solder bridge problem, connector terminals are designed to provide for solder flow more readily over the surface area of the connector terminal. However, improved flow or wetting causes solder to travel up the connector terminal such that the solder contaminates the terminal contact area, generally plated with a noble metal, in the area of electrical connection with an electronic module.
It has not heretofore been possible to adequately reduce the effect of all of these problems simultaneously. Locating adjacent terminals farther apart on the printed circuit board in order to obviate the solder bridging problem is unfeasible since the goal in the connector industry is toward more densely packed electronic modules which must be connected to a printed circuit board. Strict control of the volume of solder used and close monitoring of the soldering process has been found to be costly and unreliable. Also, known pre-solder cleaning techniques and platings and corrosion resistance coatings have also been found to be costly and unreliable.
Thus, there is a long and unfilled need in the prior art for an improved surface mount connector for a high-density leaded or leadless electronic module that allows for simple and effective solder connection of the connector terminals to a printed circuit substrate. The present invention provides a surface mount connector which satisfies this need.